The present invention relates to fasteners in general and, more in particular, to counterbored internally threaded fasteners such as collars or nuts.
Threaded fasteners have been used a long time to fasten workpieces together to form joints. (Threaded fasteners go by different names; internally threaded fasteners can be "nuts" or "collars;" externally threaded fasteners can be "bolts" or "studs;" in this specification all will be subsumed under "nuts" and "externally threaded fasteners" or "bolts.") A nut and bolt fastens workpieces together to form a joint by compressive engagement between the nut on one side of the workpieces and the head of the bolt on the opposite side. A nut has an internal thread that engages an external thread of a bolt to effect this engagement. A washer between a nut and the workpiece can protect the workpiece from being damaged by the nut as the nut rotates during tightening on the bolt; it does this by reducing and making more uniform friction acting on the workpiece. A washer can also distribute the compressive load of a nut evenly on a workpiece and can reduce the unit loading or pressure that the nut applies to the workpiece. To effect their respective functions, the hardness of washers can be different from the hardness of the nuts they are used with, usually the washers are harder.
A nut often has a counterbore between its thread and one axial end of the nut. The counterbore provides a space to accommodate thread convolutions of a bolt so that only a smooth shank of the bolt is within the workpieces fastened even though the workpieces have different total thickness; in other words, the same bolt can be used with different thickness workpieces without having its thread within the workpieces. (The avoidance of threads within the workpieces has substantial structural advantages.)
Cross-threading has plagued threaded fasteners. The thread of one fastener is suppose to be in the helical space between the thread convolutions of its mating fastener. Cross-threading occurs when the thread of one fastener does not align with the thread of the mating fastener; when the two threads are not in proper relationship to each other. When this occurs, the thread of one fastener cuts through the thread of its mating fastener and the integrity of the resulting joint is lost, or at least substantially prejudiced.
Cross-threading occurs when the fasteners are not in axial alignment when engagement is attempted during installation. The likelihood of cross-threading increases where fastener installation is not sufficiently sensitive to the resistance that occurs at the outset of crossthreading. Power wrenches, for example, apply high starting torques that result in cross-threading unless the fasteners are properly aligned.
Sometimes assembly of nuts and bolts with workpieces may be facilitated by initially hanging several nuts on the bolts that they will be used with and tightening later. This can be difficult in some environments. For example, in environments where gravity cocks the nuts with respect to the bolts hanging can produce cross-threading.